I reckon an average one would fetch around £150 possibly, but no more than £250 this is just as it comes unchecked. Some people have paid as little as £75, my own engines I have paid no more tha £35 for them, but it just depends if you are buying it off someone who knows the price that they go for. As they say, it really is how long is a piece of string!
I recently bought unchecked engine for €200. Rusted and corroded but main parts are usable when restored. And there are very few M20 engines available in my area. Just for information.
If anyone is interested. I have this 1943 WM20 engine for sale. It's complete a turns over fine, but I haven't inspected it. The mag platform has only very slight wear. One broken fin at top of barrel and a couple on head, inboard so they don't show to much. Looking for around £250.
Also I have a 1940 WM20 crankcase and flywheel assembly for sale. This was in my bike till just recently. It has a good big end and was fitted only a few hundred miles ago with new British main bearings and a genuine plus 20 thou piston. £150. Located in South UK.
More pictures of either on request. Ron